Optical conductors are commonly used in telecommunications. Optical fibers based on silica generally conveyed data at a wavelength approximately in the range 1300 nanometers to 1550 nanometers. Such an optical fiber is formed of an optically-active portion constituted by a core that conveys most of the light wave, and by cladding, the core and the cladding having different refractive indices. Such a fiber usually also has an outer peripheral portion that is not optically active and that is referred to as the "outer sheath" which, together with the cladding, forms the sheath of the optical fiber. Since an optical fiber is drawn down from a preform which is geometrically similar in section, the same terms "core", "cladding", and "outer sheath" are used for the preform from which the optical fiber is manufactured. Each fiber is protected by covering made of a polymer material, and such protective covering is often covered with other covering made of a colored polymer. A set of optical fibers may be assembled together to form a ribbon. In which case, the material holding together the fibers of the ribbon, which material is referred to as the "matrix", is also a polymer. In a telecommunications cable, the individual optical fibers or the ribbons of optical fibers are generally disposed in a tube made of metal or of a plastics material.
It is known that optical fibers must not be exposed to hydrogen because it degrades their transmission properties. The higher the hydrogen partial pressure to which the fiber is subjected, the worse the degradation. Hydrogen comes, in particular, from decomposition of the polymers of which the fiber covering or ribbon matrix is made. Hydrogen can also come from decomposition of the filler substance generally disposed in the cable in order to hold the fibers in the tube containing the optical fibers, and to prevent moisture from advancing in the event that the tube is broken or damaged. Such types of decomposition occur naturally due to aging.
Patent GB-B-2,145,240 mentions manufacturing optical fiber by performing fiber-drawing on an optical preform, and it describes said optical fiber comprising an optically-active portion made up of a core and of cladding, with the core and the cladding having different refractive indices, and of an outer sheath which, in addition to silica, contains a dopant belonging to a list of oxides. Among the list of oxides, alumina is mentioned and boron oxide is preferred. Neither the method of manufacturing the preform nor the structure of such a preform are specified. The structure of the final optical fiber is given by way of example and shown in a figure. That optical fiber comprises a core of radius R, made of doped silica, and of diameter 50 .mu.m, corresponding to 2 R, and, around the core, a sheath that takes the diameter of the optical fiber to 125 .mu.m, i.e 5 R, and that comprises cladding taking the diameter to 90 .mu.m, i.e. 3.6 R, and an outer sheath, of thickness equal to 17.5 .mu.m, i.e. 0.7 R. The concentration of oxide in said outer sheath preferably lies in the range 1% to 20% by weight of oxide relative to the composition of the outer sheath.
The presence of a large quantity of alumina in the outer sheath formed in that way makes it problematic to manufacture optical fiber from the preform under suitable conditions, in particular in terms of diameter stability during fiber-drawing, because the viscosity of the doped outer sheath is too low. With only 1% by weight of Al.sub.2 O.sub.3 in silica, it is difficult to remain in the generally accepted tolerance bracket which is 125.+-.0.5 .mu.m, and the tolerance becomes equal to 125.+-.1.5 .mu.m. That makes it problematic to connect the optical fibers together.
Furthermore, it is important for the outer sheath obtained to offer good performance in terms of mechanical properties, and an outer sheath that is too thick, such as the sheath described in Patent GB-B-2,145,240 can cause a reduction in performance, in particular when subjected to traction testing.